The present invention relates to a preamplifier noise filtering circuit. More particularly, the present invention relates to a preamplifier circuit that permits rapid transitions between operating states thereof.
As magnetic disk drive design has evolved, track widths for data storage in the magnetic media have become narrower. As track widths have become narrower, it has become increasingly difficult to combine the media magnetization retrieval (read) and media magnetizing (write) functions in a single thin film transducer. Accordingly, disk drive designers have begun to use separate read and write transducer elements. Typically, the write element comprises a thin film transducer. The read element typically comprises a magnetoresistive (MR) sensor or head. The MR head is formed from a strip of magnetoresistive material, typically permalloy, which has a resistance which varies with the magnitude of magnetic flux passing through that strip.
An MR sensor positioned adjacent a magnetic data storage disk can be used to sense the magnetic fields associated with locally magnetized regions in the magnetic media on the disk along the tracks. Such localized regions represent data through the alternative magnetizations thereof, the sensor reading these magnetizations as the disk is rotated by it.
A known operating and sensing circuit having an MR head is illustrated in FIG. 1. The magnetoresistive head is connected across the emitters of a NPN transistor pair. A pair of high impedance resistors are each connected at an end thereof to a corresponding one of the junctions of the emitters of the transistor pair and the MR head. At the opposite ends, each of the resistors is connected to the inverting input of a corresponding high gain amplifier having its noninverting input grounded, and its output connected to the base of a corresponding one of the transistor pair to form two negative feedback loops to bias these transistors. Each of another pair of resistors is connected between a corresponding one of the collectors of the transistor pair and a voltage source. A differential input-differential output amplifier has each input thereof connected to a corresponding one of these resistors and each output connected to a corresponding one of these transistor bases to keep the voltages across these resistors substantially equal. The differential input-differential output amplifier cannot respond quickly enough to the changes in voltage across the MR head when reading disk local region magnetizations in the read state at sufficiently high frequencies to maintain the voltages across the resistors perfectly equal. Thus, magnetization changes between disk local magnetization regions will lead to decaying pulses between the operating and sensing circuit outputs representing the data stored on the disk.
The operating and sensing circuit is also used to bias the MR head. Current is drawn from the operating and sensing circuit by a current source connected to one of the emitters of the transistor pair to provide in effect the head bias current, actually drawing twice the head current in order to permit equalization of the transistor pair collector currents. Any noise generated by this current source thus appears directly across the MR head which is amplified in the same proportion as disk local region magnetization change signals, and such noise may in some situations be of a nature so similar to those signals to result in data reading errors. One possibility for eliminating the noise generated by this current source is to filter the noise in the read pass band by using a filter with a large capacitive shunting characteristic at the current source shown as a single large capacitor in FIG. 1.
However, when a transition from an operating and sensing circuit power saving (idle) state to a preamplifier retrieval readiness (read) state is desired, the delay time to charge such a filter or capacitor to thus complete a change in the circuit states would be unacceptable to many users of magnetic storage devices. The present invention provides a circuit to allow for fast idle state-to-read state transitions in an operating and sensing circuit when using a magnetoresistive head while retaining the capability of filtering out noise due to the current source required to bias the MR head.